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Californio Samba Member
Joined: May 17, 2007 Posts: 1301
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Posted: Tue Aug 16, 2011 9:48 am Post subject: Possible to test fan resistance with an ohm meter? |
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I have an inexpensive multi-meter and would like to test the current draw on a used fresh air blower motor. But I think it will draw too many amps for the meter if I just hook it up in series. Can the ohm testing function be of any use here? How would you test the current draw if the max on your meter is 10 Amps? |
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MarkWard Samba Member
Joined: February 09, 2005 Posts: 17109 Location: Retired South Florida
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Posted: Tue Aug 16, 2011 10:30 am Post subject: |
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You'd want to check the amp draw. You could read the resistance of the used blower motor with the ohm setting, but would need to know the spec of a good motor. You might be able to make a test harness out of a inline fuse holder. Start with a 5 amp fuse if it blows, move up to the next amp fuse.
If you use your meter inline, it should have an internal fuse to protect it if you exceed the 10amp draw. The size of the stock fuse in the vanagon should indicate the max amp draw of the circuit. Don't have my Bentley hand to check the stock fuse size. _________________ ☮️ |
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dhaavers Samba Member
Joined: March 19, 2010 Posts: 7753 Location: NE MN (tinyurl.com/dhaaverslocation)
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Posted: Tue Aug 16, 2011 11:05 am Post subject: |
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Fuse #13: blower motor = 20A _________________ 86 White Wolfsburg Westy Weekender
"The WonderVan"
<EDITED TO PROTECT INNOCENT PIXELS> |
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Californio Samba Member
Joined: May 17, 2007 Posts: 1301
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Posted: Tue Aug 16, 2011 11:10 am Post subject: |
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That's a good practical way to test it. Basically, if it blows a 20 amp fuse, I don't want it.
But in principle, I'm right that there is a relationship between ohms and current draw, and that theoretically the measurements should both be accurate?
Still trying to wrap my mind around this electrical stuff. |
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MarkWard Samba Member
Joined: February 09, 2005 Posts: 17109 Location: Retired South Florida
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dhaavers Samba Member
Joined: March 19, 2010 Posts: 7753 Location: NE MN (tinyurl.com/dhaaverslocation)
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Posted: Tue Aug 16, 2011 11:19 am Post subject: |
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Inverse function:
ohms=measure of resistance to current (ie: "resistor")
amps=measure of current allowed
Can't measure both with just one circuit.
Why not just shell out a $20 for a better tester?
At least you'll be measuring what you need to measure... _________________ 86 White Wolfsburg Westy Weekender
"The WonderVan"
<EDITED TO PROTECT INNOCENT PIXELS> |
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presslab Samba Member
Joined: September 29, 2008 Posts: 1730 Location: Sonoma County
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Posted: Tue Aug 16, 2011 11:27 am Post subject: |
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Theoretically, yes, you can measure the resistance. The problem is that it will be very low - about 0.1 ohms. A special 4-wire ohmmeter is needed to measure this low with any accuracy and these things aren't cheap or portable.
It's more complicated than Ohm's Law, so it's not a simple calculation. The sum of the back-EMF of the motor and the IR of the windings will equal the applied voltage. Ignoring eddy currents in the core, of course.
Measuring the current is the only practical way to know the condition of the motor.
Sears has a cheap clamp-on DC current meter. _________________ 1986 Vanagon Westfalia EJ25
1988 Subaru GL-10 EJ20G --- 2000 Honda XR650L
2010 Titus El Guapo --- 2011 On-One 456 Ti |
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